Basic Concepts & Short Tricks

Duration: 9 min

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This educational video is a lecture on calendar problems, specifically focusing on determining the day of the week for a given date. The instructor, Yash Jain Sir, begins by posing the question: "On what day does 21 June 1984 lie?" He then systematically breaks down the problem using a step-by-step method. The first step involves calculating the total number of days from a known reference point, which is 1984. He explains that a year has 365 days, a leap year has 366, and a week has 7 days. He calculates the total days from 1984 to the present, noting that 1984 is a leap year. The second step is to find the remainder when the total number of days is divided by 7, which gives the number of odd days. The video then transitions to a section on how to identify leap years, explaining the rule: a year is a leap year if it is divisible by 4, but if it is a century year (ending in 00), it must be divisible by 400. The final part of the video covers a method for finding the day of the week for a future date, using a table that assigns a code to each day of the week (e.g., Monday = 1, Tuesday = 2, etc.). The instructor demonstrates this by calculating the day of the week for the 23rd day of a month, given that the 1st is a Monday. The video concludes with a summary of the key concepts and a thank you message.

Chapters

  1. 0:00 – 2:00 00:00-02:00

    The video opens with a title card showing a calendar and the word "CALENDAR". The main content begins with a question displayed on a green banner: "On what day does 21 June 1984 lie?". The instructor, Yash Jain Sir, is visible in a small window in the bottom right corner. The background is a blackboard with various educational doodles. The instructor introduces the topic of calendar problems and states that the video will solve the question in 60 seconds. He begins by explaining the basic components of a calendar: Year, Months, Weeks, and Days. He writes "Year" on the board and explains that a year has 365 days, which is an ordinary year. He then writes "Months" and states there are 12 months. He writes "Weeks" and says there are 52 weeks. Finally, he writes "Days" and explains that a week has 7 days. He also mentions that a leap year has 366 days. The instructor then begins to calculate the total number of days from 1984 to the present, starting with the year 1984.

  2. 2:00 – 5:00 02:00-05:00

    The instructor continues the calculation of the total number of days. He writes "365" on the board, representing the number of days in an ordinary year. He then writes "366" for a leap year. He explains that to find the day of the week, we need to find the remainder when the total number of days is divided by 7. He writes "365 / 7 = 52 weeks and 1 odd day". He then explains that a leap year has 366 days, which is 52 weeks and 2 odd days. He writes "366 / 7 = 52 weeks and 2 odd days". He then calculates the total number of odd days from 1984 to the present. He writes "1984" on the board and explains that 1984 is a leap year. He then calculates the number of odd days from 1984 to 2023. He writes "1984" and then "2023". He calculates the number of years between 1984 and 2023, which is 39 years. He then calculates the number of leap years in that period. He writes "39" and then "10". He explains that there are 10 leap years in that period. He then calculates the total number of odd days. He writes "39 * 1 = 39" and "10 * 2 = 20". He adds these together to get 59. He then divides 59 by 7 to get the remainder, which is 3. He writes "59 / 7 = 8 weeks and 3 odd days". He then explains that the day of the week for 21 June 1984 is 3 days after the day of the week for 1 January 1984. He then moves on to the next part of the video.

  3. 5:00 – 9:03 05:00-09:03

    The video transitions to a new section titled "Helpful Tips". The instructor explains how to determine if a year is a leap year. He writes the rule on the board: "A year is a leap year if it is divisible by 4, but if it is a century year (ending in 00), it must be divisible by 400." He gives examples: 1996 is a leap year because 1996 / 4 = 499. 2019 is not a leap year because 2019 / 4 = 504.75. 1600 is a leap year because 1600 / 400 = 4. 1700 is not a leap year because 1700 / 400 = 4.25. The instructor then moves on to the next topic: finding the day of the week for a future date. He writes a table on the board that assigns a code to each day of the week: Monday = 1, Tuesday = 2, Wednesday = 3, Thursday = 4, Friday = 5, Saturday = 6, Sunday = 7. He then gives an example: "If today is Monday, then what will be the day on 23rd?" He explains that the 23rd is 22 days after the 1st. He calculates 22 / 7 = 3 weeks and 1 odd day. He then adds 1 to the code for Monday (1 + 1 = 2), which corresponds to Tuesday. He then moves on to the next topic: the number of odd days in a century. He writes a table on the board that shows the number of odd days in different centuries: 100 years = 5 odd days, 200 years = 3 odd days, 300 years = 1 odd day, 400 years = 0 odd days. He explains that this is because 400 years have 97 leap years and 303 ordinary years, which gives a total of 365*303 + 366*97 = 146097 days, which is exactly 20871 weeks. The video ends with a thank you message.

The video provides a comprehensive, step-by-step guide to solving calendar problems. It begins with a clear problem statement and systematically breaks down the solution into logical steps. The instructor first establishes the fundamental concepts of days, weeks, and years, then introduces the concept of 'odd days' as the key to solving the problem. The method involves calculating the total number of odd days from a reference year to the target year and using the remainder to determine the day of the week. The video also covers essential prerequisite knowledge, such as the rules for identifying leap years and the number of odd days in a century. The use of a table to assign codes to days of the week provides a practical method for calculating future days. The overall structure is logical and pedagogically sound, making it an effective resource for students preparing for competitive exams.